1. Field of the Invention
The present invention relates to a test structure for reliability evaluation of an insulation film such as a gate insulating film.
2. Description of the Background Art
Generally, a technique called xe2x80x9cTime Depend Dielectric Breakdown (TDDB)xe2x80x9d is used for reliability evaluation of an insulation film such as a gate insulating film. FIG. 28 is a cross-sectional view of a conventional test structure for insulation-film evaluation used in the TDDB test. A gate insulating film 102 to be evaluated is formed across the surface of a substrate 101 and a gate electrode 103 is formed across the surface of the gate insulating film 102. The TDDB test is conducted using an evaluation pattern such as a pattern including the edge of the active region (field edge), a pattern including the gate edge, and a flat pattern consisting only of surface components, thereby evaluating process damage to the gate insulating film with respect to each component. For instance, the TDDB test using a gate edge pattern evaluates how the gate insulating film receives damage from etching in forming a gate electrode.
The TDDB test for a certain period of time causes a dielectric breakdown in the gate insulating film at any rate. The reliability of the gate insulating film is evaluated by means of statistical processing of such a fault-time distribution. To clarify the physical cause of failures, however, failure analysis is absolutely necessary.
According to a failure analysis technique using the conventional test structure for insulation-film evaluation, failure analysis is performed in the following two steps: (1) finding the physical location of a failure using an emission analyzer or an electron beam tester, for example; and (2) observing the failure using a scanning electron microscopy (SEM), for example. This technique has a problem that especially the former step is complicated.
A first aspect of the present invention is directed to a test structure for insulation-film evaluation comprising: a substrate; an insulation film to be evaluated which is formed on a main surface of the substrate; an electrode array formed on the insulation film and consisting of a plurality of electrodes arranged in spaced manner, a plurality of wires for applying a voltage to the plurality of electrodes, the voltage including a voltage for applying evaluation stress to the insulation film and a voltage for sequentially transferring charge generated in breakdown part of the insulation film along the electrode array to detect the presence or absence of breakdown in the insulation film due to the application of the evaluation stress; and a read circuit for sequentially reading the presence or absence of the transfer of the charge.
According to a second aspect of the present invention, in the test structure of the first aspect, adjacent three of the electrodes in the electrode array are connected to different ones of the wires.
According to a third aspect of the present invention, in the test structure of the first aspect, the electrode array consists of the electrodes under which an impurity region is formed in the main surface of the substrate and the electrodes under which the impurity region is not formed in the main surface of the substrate, which are aligned alternately.
According to a fourth aspect of the present invention, in the test structure of the first aspect, the electrode array consists of the electrodes doped with impurities and the electrodes not doped with the impurities, which are aligned alternately.
According to a fifth aspect of the present invention, in the test structure of the first aspect, adjacent four of the electrodes in the electrode array are connected to different ones of the wires.
According to a sixth aspect of the present invention, in the test structure of the first aspect, the electrode array consists of first electrodes and second electrodes which are aligned alternately, the first electrodes formed on a thin-film portion of the insulation film having a first thickness, the second electrodes formed on a thick-film portion of the insulation film having a second thickness larger than the first thickness; and adjacent four of the electrodes in the electrode array are connected to different ones of the wires.
According to a seventh aspect of the present invention, in the test structure of the sixth aspect, the second electrodes extend over the end portions of the first electrodes.
According to an eighth aspect of the present invention, in the test structure of the first aspect, a plurality of impurity regions with different impurity concentrations are adjacently formed in the main surface of the substrate under each of the electrodes.
According to a ninth aspect of the present invention, the test structure of either of the first to eighth aspects further comprises: a drawn region formed along the electrode array in the main surface of the substrate, for drawing charge flowing out of a potential well under breakdown part of the insulation film caused by the application of the evaluation stress.
According to a tenth aspect of the present invention, in the test structure of either of the first to ninth aspects, the substrate includes an element isolation region where an clement isolation insulation film is formed, and an active region defined by the element isolation insulation film; and the end portions of the electrodes are within the active region.
According to an eleventh aspect of the present invention, in the test structure of either of the first to ninth aspects, the substrate includes an element isolation region where an element isolation insulation film is formed, and an active region defined by the element isolation insulation film; and the electrodes extend over a boundary between the element isolation region and the active region.
According to a twelfth aspect of the present invention, in the test structure of either of the first to eleventh aspects, cells including the electrodes are arranged in the form of a matrix, all of the cells being electrically connected to each other by an active region of the substrate.
According to a thirteenth aspect of the present invention, in the test structure of either of the first to eleventh aspects, cells including the electrodes are arranged in the form of a matrix, a plurality of the cells being divided into a plurality of groups by an element isolation region of the substrate; and the read circuit is provided for each of the groups.
A fourteenth aspect of the present invention is directed to a test structure for insulation-film evaluation, comprising: a substrate; an insulation film to be evaluated which is formed on a main surface of the substrate; an electrode matrix consisting of a plurality of electrodes formed in the form of a matrix on the insulation film; a stress applying circuit for applying evaluation stress to the insulation film; a first shift register capable of applying a voltage to the plurality of electrodes for each column of the electrode matrix; and a second shift register which is electrically connected to the substrate for each row of the electrode matrix and capable of detecting whether or not the application of the voltage by the first register causes current flow to the substrate through the insulation film.
According to a fifteenth aspect of the present invention, in the test structure of the fourteenth aspect, the electrode matrix includes at least two rows having the insulation film of different thicknesses.
According to a sixteenth aspect of the present invention, in the test structure of either of the first to fifteenth aspects, cells including the electrodes are arranged in the form of a matrix; and a mark is provided at predetermined blocks on the substrate.
The test structure for insulation-film evaluation of the first aspect adopts the CCD structure, wherein the charge supplied into the substrate due to a breakdown of the insulation film is sequentially transferred so that the read circuit can locate a failure. This structure simplifies failure location in the insulation film as compared to a conventional technique of locating failures using an emission analyzer, for example.
In the test structure of the second aspect, three-phase charge transfer allows failure location in the insulation film.
In the test structure of the third aspect, the impurity regions formed in the main surface of the substrate provides the potential difference between adjacent cells. This enables two-phase charge transfer.
In the test structure of the fourth aspect, the impurities introduced into the electrodes provides the potential difference between adjacent cells. This enables two-phase charge transfer.
In the test structure of the fifth aspect, the voltage of different magnitudes, which is applied from the outside to the electrodes through the wires, provides the potential difference between adjacent cells. This enables two-phase charge transfer.
In the test structure of the sixth aspect, the insulation film of different thicknesses provides the potential difference between adjacent cells. This enables two-phase charge transfer.
The test structure of the seventh aspect eliminates a potential barrier to be an impediment to charge transfer.
In the test structure of the eighth aspect, the plurality of impurity regions with different impurity concentrations provides the potential difference. Accordingly, the charge supplied from breakdown part of the insulation film into the substrate can sequentially be transferred by simple two-phase operations using a two-input system. This allows the read circuit to find the locations of failures.
In the test structure of the ninth aspect, even if too much charge is supplied from breakdown part of the insulation film into the substrate and flows out of the potential well under the breakdown part, the overflowing charge can be drawn to the drawn region. This improves accuracy in failure location in the insulation film.
The test structure of the tenth aspect enables the TDDB test using a pattern including the gate edge.
The test structure of the eleventh aspect enables the TDDB test using a pattern including the field edge.
The test structure of the twelfth aspect requires only one read circuit. This results in simplification of the structure.
In the test structure of the thirteenth aspect, failure location can independently performed for each group. This improves accuracy in failure location.
In the test structure of the fourteenth aspect, a plurality of electrodes are arranged in the form of a matrix. The row and column of the electrode matrix in which the insulation film suffers a breakdown can be detected by varying combinations of selections by the first shift register and the second shift register. This simplifies specification of the locations of failures in the insulation film.
The fifteenth aspect allows a plurality of insulation films of different thicknesses to be evaluated using a single test structure.
In the test structure of the sixteenth aspect, for failure observations using an SEM, an operator can look to a mark on the test structure to recognize the locations of failures in the insulation film. This simplifies the operation of entering the detected failures within the visual field of the SEM.
An object of the present invention is to provide a test structure for insulation-film evaluation, which allows simple failure location.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.